1. Field of the Invention
The present invention relates to a solid-state image pickup element, its driving method thereof, and a control program and, more particularly, to a CMOS solid-state image pickup element, its driving method thereof, and a control program.
2. Description of the Related Art
Recently, an image pickup element is being advanced in such a manner that it has the large number of pixels, while this advance rises a problem that higher read-out time is required.
Multi-channelization can be mentioned as one of well-known techniques for high-speed read-out of the image pickup element. The multi-channelization is such a technique that a plurality of output terminals are provided so that groups of vertical output lines output signals in parallel to the different output terminals, respectively, for example, thereby enabling the high-speed read-out to be performed.
However, such multi-channelization does nothing but shorten a time required for horizontal transfer in a driving method of the image pickup element. Therefore, such a time is hardly shortened with respect to the operations during so-called a horizontal blanking period, which need to be executed before the horizontal transfer. Such operations are such that, for example, a signal electric charge is transferred from a photoelectric conversion element to a floating diffusion layer portion, signal potentials appearing on the vertical output lines are transferred to capacitors serving as signal storage units arranged on the column basis, and the like.
On the other hand, a technique for the high-speed read-out which is attained by simultaneously performing a part of the operation to be performed during the horizontal blanking period and the horizontal transfer is also known besides the multi-channelization.
According to the Official Gazette of Japanese Laid-Open Patent Application No. 2010-206653 (Patent Literature 1), a construction and a driving method of an image pickup element which can realize such a technique are disclosed. That is, there is disclosed an image pickup element “comprising: a plurality of pixels arranged two-dimensionally; a plurality of vertical output lines to which a plurality of pixels arranged every other predetermined rows in each column of the plurality of pixels are connected, respectively; a first signal storage unit for accumulating pixel signals transferred from the pixels by the plurality of vertical output lines; a second signal storage unit for accumulating the pixel signal which is transferred from the first signal storage unit; and a horizontal transfer unit for horizontally transferring the pixel signal accumulated in the second signal storage unit, wherein while the pixel signals are transferred from the pixels to the first signal storage unit by the plurality of vertical output lines, the pixel signals which have already been accumulated in the second signal storage unit are horizontally transferred by the horizontal transfer unit”.
According to the above Patent Literature, since the period of time during which the pixel signals are transferred from the pixels to the first signal storage unit, that is, a period which covers a predetermined portion of the horizontal blanking period, and the horizontal transfer period can be simultaneously parallelized, the high-speed read-out can be performed. The “period which covers a predetermined portion” is called “first horizontal blanking period” hereinbelow.
However, according to the above Patent Literature, nothing is disclosed about a transfer of the pixel signals from the first signal storage unit to the second signal storage unit, and such a transfer cannot be parallelized with any one of the horizontal transfer and the transmission to the first signal storage unit. Therefore, even if the technique of the above Patent Literature is used, a period obtained by subtracting the first horizontal blanking period from a horizontal blanking period in the related art (such a period is hereinbelow called “second horizontal blanking period” and it coincides with a transfer period of the pixel signals from the first signal storage unit to the second signal storage unit) remains as read-out time simply added for the number of vertical lines.
On the other hand, a technique for shortening the horizontal blanking period itself has also been proposed. According to the Official Gazette of Japanese Laid-Open Patent Application Laid-Open No. 2004-134867 (Patent Literature 2), a driving method of an image pickup element which can realize such a technique is disclosed. That is, there is disclosed “a solid-state image pickup apparatus constructed by arranging a plurality of unit cells each having a plurality of photoelectric conversion elements each for converting an optical signal into a signal electric charge to be accumulated and one amplification element which receives the signal electric charge of each of the photoelectric conversion elements and outputs a signal corresponding to the signal electric charge, wherein the solid-state image pickup apparatus has a read-out circuit system which is connected to an output line of the amplification element and inputs a signal that is output from the unit cell through the output line, and the read-out circuit system has a differentiation unit which inputs a first signal corresponding to a reset level of an input unit of the amplification element, a second signal which is provided by adding a signal corresponding to the signal electric charge accumulated in at least one of the plurality of photoelectric conversion elements to the first signal, and a third signal which is provided by adding a signal corresponding to the signal electric charge accumulated in at least one of the plurality of photoelectric conversion elements to the second signal and obtains differences among a plurality of combinations of any two of the three input signals.
According to the above Patent Literature, since the input unit of the amplification element is shared by the plurality of photoelectric conversion elements, the input unit of the amplification element is not reset for a plurality of number of times, each time the signal corresponding to the signal electric charge of each element is transferred, and the signal corresponding to the reset level can be also used in common. Therefore, such an advantage that a time for resetting the input unit of the amplification element and a period for transferring the signal corresponding to the reset level after the resetting can be reduced by a time corresponding to the number which is the number of photoelectric conversion elements which share the input unit of the amplification element−1 is obtained.
However, according to the above Patent Literature, the horizontal blanking period and the horizontal transfer period are perfectly separated with respect to the time and such a construction that they can be parallelized as disclosed in Patent Literature 1 is not provided. Therefore, with respect to a point of attainment of the high-speed read-out, its effect lies only within a range of a shortening of the horizontal blanking period.
Moreover, in order to realize the further high-speed read-out by combining the simultaneous parallelization of the first horizontal blanking period and the horizontal transfer period (Patent Literature 1) and the shortening of the horizontal blanking period (Patent Literature 2), there are the following problems in which they cannot be simply combined. That is, in the case of applying the driving method of the image pickup element according to Patent Literature 2 to the construction of the image pickup element and its driving method according to Patent Literature 1, the transfer from the first signal storage unit to the second signal storage unit and the transfer of the added signals from the photoelectric conversion elements to the first signal storage unit cannot be simply parallelized. This is because it is necessary to provide the capacitors of the number of added signals+1 (+1 is a necessary number of capacitors to accumulate the signal corresponding to the reset level) as a first signal storage unit. The reason is that when the signal is transferred from the first signal storage unit to the second signal storage unit (during the second horizontal blanking period), a new added signal cannot be transferred to the two first signal storage units regarding a differentiation processing accompanied with the horizontal transfer which is executed just after the transfer period. If the driving method of the image pickup element according to Patent Literature 2 is combined, in spite of a fact that the larger the number of added signals is, the higher the effect of the shortening of the horizontal blanking period is, it is accompanied with an increase in the number of capacitors of the storage unit. This results in an increase in chip area. In such a situation that a space on the chip is decreased in association with the pitch-narrowing in the pixel column direction caused by fine scaling due to the large number of pixels, a difficulty occurs in a layout itself of the capacitors of the storage unit.